JP6745516B2 - Optical element - Google Patents

Description

TECHNICAL FIELD The present invention relates to an optical element that causes light to be refracted, separated, and condensed.

Lenses, prisms, and the like are extremely widely used as optical elements that realize refraction of light, polarization separation, and light collection. Most of them have a three-dimensional shape like a convex lens and a concave lens, and are manufactured as one function one by one, so that integration and miniaturization are often difficult. In recent years, a technique of finely processing the surface of a transparent substrate, changing the phase of the light beam transmitted perpendicularly to it and tilting the wavefront to manipulate the propagation after transmission (called a gradient metasurface) ) Is making progress.

It is not uncommon for the amount of wavefront deformation required at that time to be several times or several tens of times the wavelength. On the other hand, since the phase change amount of the light passing through the surface is practically a fraction to several times 2π radians, it is necessary to perform an operation to return the phase change amount to zero in a sawtooth wave every 2π radians. ..

The above-described operation of returning the amount of phase change to zero in a sawtooth manner every 2π radians inevitably causes scattering of light near the discontinuity point and accompanying errors in amplitude and phase.
The following means is known as a method for reducing it (Non-Patent Document 1).
That is, minute half-wave plates having various orientations are arranged on the substrate surface without gaps in each region (A).
(B) The property that the phase shift that circularly polarized light undergoes when passing through that region is equal to twice the angle θ formed by the principal axis with respect to a certain reference direction is used.
More specifically, the electric field of the incident light in FIG. 1 is, for example, E _x =E ₀ cos(ωt), E _y =E ₀ sin(ωt)
If it is a circularly polarized light given by, the ξη axis is taken as shown in Fig. 1, and if a ½ wavelength plate having a principal axis in the ξη axis direction is inserted, the light after passing becomes circularly polarized light in the opposite direction and the relative phase is 2θ. It is known that only changes (Non-Patent Document 1).

When it is necessary to change the phase transition continuously over 2π, for example, θ should be defined as shown in the upper part of Fig. 1, and θ should be continuously changed over π. The phase angle can be changed by any number of times 2π without discontinuity by changing the value by several times of π. If θ approximately linearly increases or decreases with x, the transmitted circularly polarized wavefront undergoes a linear transformation with respect to x, and a prism action occurs.

The necessary "minute half-wave plate having various orientations in each region" is realized by periodically arranging deep grooves in the substrate. An infinitely long groove array formed periodically on a solid surface causes a larger phase lag for polarized light whose electric field is parallel to the groove and polarized light whose electric field is perpendicular to the groove. In the half-wave plate, it is necessary to match the phase difference to π, and for design and processing reasons, the spacing between grooves is often about 1/3 wavelength to 1/2 wavelength, and 1/4 wavelength. Never become.

D. Lin, P. Fan, E. Hasman and M. Brongersma,”Dielectric gradient metasurface optical elements, Science, Applied Optics, 18 July 2014, pp. 298-302. N. Yu and F. Capasso, “Flat optics with designer metasurfaces”, Nature materials, 23 January 2014, pp.139-149.

Japanese Patent No. 3325825 "Three-dimensional periodic structure and its manufacturing method and film manufacturing method"

The above-mentioned method using the half-wave plate by surface processing has the following difficulties.

(1) The interval between the grooves or the period of the periodic grooves is at least ⅓ wavelength or more. In order to control the light beam, it is desired to precisely control the phase for each place, but it is limited by the groove spacing of the wave plate. Actually, in order for the groove to function as a wave plate and to have a principal axis direction different from that of the adjacent region before that, it is necessary that the length of the groove is at least equal to or more than the interval between the grooves, preferably twice or more. The size of the micro area cannot be sufficiently small. This will be described below. Of the regions D in FIG. 1, the one in which the length of the groove in the region is the minimum is represented by the symbol d. Similarly, in FIG. 5, the symbol d is also defined. Further, the period of the groove that is periodically repeated (also referred to as “inter-groove unit period”) is represented by the symbol p. In order to operate as a half-wave plate, d/p needs to be large to some extent. When d/p is finite, the phase difference due to birefringence in that region is smaller than π and is estimated to be about π(1-p/2d). In order that the phase difference which should be originally π is, for example, 0.95π or more, or 0.9π or more, or 0.75π or more, or 0.5π or more, d is 10 p or more, 5 p or more, 2 p or more, respectively. It must be p or more.

On the other hand, d should be kept small for higher definition. In the prisms of FIGS. 1 and 5, the upper limit of d is determined by the requirements of the element, and the smaller the value, the higher the element performance (because the quantization error is small). On the other hand, since p is required to be smaller by one digit to half digit than that, the advantage of being able to reduce p is great.

Further, when the grooves are curved as shown in FIG. 2, if the same curves are arranged at equal pitches, the pitch becomes narrower as it approaches the vertical, and it is necessary to keep the pitch by reducing (thinning) the number of grooves. There is. Even in such a case, the pitch interval cannot be strictly fixed, the pitch interval varies from place to place, and the phase difference deviates from the half-wave plate.

(2) It is necessary to form an antireflection layer on the surface in order to avoid unnecessary reflection of light on the element surface, but it is difficult to form the film on a half-wave plate by surface processing.

(3) As an ordinary member for the optical industry, it is difficult to use an element that performs a desired operation for circularly polarized light, and it is desired to operate for linearly polarized light. In order to meet the demand, it is necessary to place a quarter-wave plate in front of and behind the device, which is troublesome. It is the easiest to use if a quarter-wave plate can be integrated in front of and behind the element in which minute areas are spread.

Therefore, the present invention has been made in view of such problems, and an object thereof is to provide a volume type optical element using a self-cloning photonic crystal instead of a meta-surface.
To summarize the effect in advance,
First, it is possible to average and smooth the influence of pattern mismatch at the sub-region boundary and discontinuity in thinning on the light by diffraction accompanying propagation in the Z direction (FIGS. 6 and 9).
Secondly, the phase difference between polarized lights can be kept uniform even if the pitch between lines becomes non-uniform or non-uniform due to the curved shape or thinning (FIG. 3).
Thirdly, the polarization purity is maintained by integrating the photonic crystal prism and the photonic crystal lens as in Example 4 described later (FIG. 14).

The main purpose of the present invention is to use a wave plate made of a self-cloning photonic crystal under a design condition in which an interval between grooves, that is, a fundamental period is sufficiently smaller than a wavelength used.

A first aspect of the present invention relates to an optical element. The optical element includes photonic crystal half-wave plates formed in the xy plane in the three-dimensional space x, y, and z and stacked in the z-axis direction. The optical element has one or a plurality of regions that are single or repeated in the x-axis direction, and the region is divided into a plurality of strip-shaped sub-regions in the x-axis direction. In the groove direction of the photonic crystal, the angle with respect to the y-axis direction changes stepwise in the range of 0° to 180° within the region, and the angle with respect to the y-axis direction within the sub-region is uniform. It will be like. Then, the optical element directs the light incident in the z-axis direction to the right-hand circularly polarized light in the direction from the z-axis toward the x-axis and the direction from the z-axis toward the −x-axis at the same angle as the certain angle. To the left-handed circularly polarized light, and is emitted after being converted and converted.

Another embodiment of the optical element will be described. The optical element includes half-wave plates of photonic crystals formed on the xy plane in the three-dimensional space x, y, z and stacked in the z-axis direction. The optical element has a single or repeated region in the x-axis direction. The groove direction of the photonic crystal is a curve, and the angle with respect to the y-axis direction continuously changes in the range of 0° to 180°. The optical element directs the light incident in the z-axis direction to the x-axis by the same angle from the z-axis to the x-axis, and the direction from the z-axis to the −x-axis at the certain angle. To the left-handed circularly polarized light, and is emitted after being converted and converted.

In the optical element having the curved groove described above, the other of the convex portion and the concave portion is branched/merged so that the ratio of the maximum value and the minimum value inside the region is 4 times or less. Are preferably arranged geometrically (see FIG. 2 etc.).

In the above-mentioned optical element having a curved groove, when the width of the region is D, the curve is preferably represented by y=(D/π)log(cos(πx/D))+constant. ..

In the optical element of the present invention, the inter-groove unit period of the photonic crystal is 40 nm or more and 1/4 or less of the wavelength of incident light, and the period in the thickness direction is 1/4 of the wavelength of incident light. The following is preferable.

In the optical element of the present invention, it is preferable that a quarter wavelength plate made of a photonic crystal is laminated or arranged on one side or both sides to separate the light incident from the z-axis direction of the optical element into two linearly polarized light beams orthogonal to each other. ..

The second aspect of the present invention relates to a composite optical element. The composite optical element has at least two optical elements described above. The two optical elements are called a first optical element and a second optical element, respectively. The first optical element and the second optical element are arranged with an interval of a certain propagation length. A quarter wave plate is provided at the subsequent stage of the second optical element. A pair of lenses having a function of condensing the linearly polarized light and diverging the linearly polarized light orthogonal to the linearly polarized light are provided at the subsequent stage of the quarter-wave plate. By including these first and second optical elements, the quarter-wave plate, and the lens, the light incident from the first optical element side can be separated into two linearly polarized light components and condensed. ..

Specifically, the first aspect of the present invention relates to an optical element. The optical element of the present invention is a wave plate whose main axis azimuth differs for each region (divided type) or a wave plate whose main axis azimuth continuously changes (curved type), and the wave plate of each region is a surface. The photonic crystal has a periodic structure inside and is laminated in the thickness direction. The photonic crystal may be formed by the self-cloning method (see Patent Document 1).

The inter-groove unit period of the in-plane periodic structure forming each wave plate and the unit period in the thickness direction of the wave plate are both ¼ or less of the wavelength of light incident on the optical element. It is preferable that the unit period between grooves of the in-plane periodic structure is 40 nm or more. The wavelength of light incident on the optical element is usually assumed to be selected from 400 nm to 1800 nm.

Further, in the wave plates of a plurality of regions, the in-plane minimum value of the wave plate groove length is not less than the inter-groove unit cycle. The upper limit of the in-plane minimum value of the wave plate groove length is preferably 50 times or less the inter-groove unit period p.

In the case of a wave plate (curved type) in which the principal axis direction changes continuously, if the pitch p of the convex portions (pitch when the pattern is a straight line) is p ₀ , then 0.5·p ₀ ≦p≦2· It is preferable that the convex portions or the concave portions are geometrically arranged such that the convex portions or the concave portions are branched and merged so as to be within p ₀ . As shown in FIG. 3, in the self-cloning photonic crystal, the change in phase difference is small with respect to the change in pitch. Therefore, it is possible to reduce the phase shift from the half-wave plate when the pitch changes.

A preferred embodiment of the optical element according to the present invention is an optical element that operates with respect to incident predetermined circularly polarized light. In this optical element, each region forms a ½ wavelength plate, and the angle of the main axis with respect to the reference direction is ½ of the amount of phase change to be given in each region.

A preferred embodiment of the optical element according to the present invention is a uniform first quarter-wave plate made of a photonic crystal on a transparent substrate, the optical element (wavefront transformation element) described above, and a photonic. A uniform second quarter-wave plate made of crystal is laminated in this order. It is preferable that the first quarter wave plate and the second quarter wave plate have different main axis directions by 90°.

In the optical element of the present invention, the self-cloning type photonic crystal wave plate is a volume type, which is fundamentally different from an inclined meta surface (for example, non-patent documents 1 and 2; It is easy to carry out a preventive treatment and to use an adhesive. Since it is a volume type, the characteristics can be kept almost constant even if the number of layers is increased and the lamination period and the in-plane period are reduced while maintaining the total thickness of the lamination, so that the structure can be made finer. ..

Another preferred embodiment of the optical element of the present invention is to change the wave plate of each region formed by parallel lines having a fixed pitch from a parallel line to a curve so as to eliminate the boundary between the regions (sub-regions). is there. By changing to a curve, the quantization error can be reduced, the resulting phase error can be reduced, the proportion of unnecessary polarization can be reduced, and the proportion of components that do not branch can be reduced.

In addition, a structure in which two uniform quarter-wave plates are sandwiched on both sides of a multi-region half-wave plate can also be manufactured by a consistent film forming process, which is excellent in terms of downsizing.

In addition, the input light is split using a multi-area 1/2 wavelength plate, and each light is input to the area-division type 1/4 wavelength plate and input to the photonic crystal lens as linearly polarized light of the same direction. Thus, an optical element capable of condensing light and removing unnecessary polarized waves can be produced by a consistent film forming process.

By making the structure finer and making it curved, it is possible to suppress light scattering and generation of unnecessary light components due to discontinuity. Further, it is excellent in workability such as surface treatment, cleaning, and adhesion treatment, and it is possible to reduce the volume, footprint, and manufacturing cost of parts.

It is a polarization grating realized by using a conventional gradient metasurface. It is a figure which shows a division type (parallel groove|channel) and a curved type comparison. It is a figure which shows the sensitivity of the phase difference with respect to a pitch change in the case of a photonic crystal. FIG. 3 is a diagram showing a band diagram of a photonic crystal and a photonic crystal with high definition. It is a figure which shows an example of the optical element (divided type) which concerns on 1st Embodiment. It is a figure which shows the comparison of the meta surface type of a prior art, and the photonic crystal type of 1st Embodiment. It is a figure which shows an example of the optical element (curved type) which concerns on 2nd Embodiment. It is a figure which shows the measured value of the principal axis direction distribution of the optical element which concerns on 1st Embodiment and the optical element which concerns on 2nd Embodiment. It is a figure which shows the measured value of the optical characteristic of the optical element which concerns on 1st Embodiment and the optical element which concerns on 2nd Embodiment. It is a figure which shows the optical element which concerns on 3rd Embodiment, Comprising: It is a figure which shows the case where 45-degree linearly polarized light enters. It is a figure which shows the optical element which concerns on 3rd Embodiment, and is a figure which shows the case where -45 degree linearly polarized light injects. It is a figure which shows the optical element which concerns on 3rd Embodiment, Comprising: It is a figure which shows the case where a 0 degree linearly polarized light enters. It is a figure which shows the optical element which concerns on 3rd Embodiment, and is a figure which shows the case where the linearly polarized light of arbitrary directions injects. It is a figure which shows an example of the composite optical element which concerns on 4th Embodiment. It is a figure which shows the lens part and coupling efficiency of the compound optical element which concerns on 4th Embodiment.

Example 1, Example 2, Example 3, and Example 4 of the present invention will be described below.

[Polarization separation element (polarization grating)]
In order for a wave traveling in the free space and traveling parallel to the z-axis to be incident on the prism and refracted by the angle α in the xz plane, the following phase change may be applied to the light beam. It is sufficient to have a phase difference of 2πsin α/λ at x=0 at x=D in FIG. 1, and the main axis of the half-wave plate has an inclination of πsin α/λ with respect to the x-axis according to the above-mentioned principle. All that needs to be done (where λ is the wavelength of light).

An example of design at a wavelength of 1550 nm will be shown with reference to FIGS. 4(a), (b), and (c).

FIG. 4A is called a band diagram or a propagation characteristic diagram of a periodic structure, and the phase difference (wave number) per unit length in the traveling direction of light that vertically penetrates the surface in the z direction is a normal value proportional to the reciprocal of the wavelength. It is a dispersion curve shown in relation to the conversion frequency. L is a basic period in the z direction (sum of thicknesses of two kinds of transparent bodies).

FIG. 4B shows a typical structure of a photonic crystal used in the optical communication wavelength band.
The specifications of the photonic crystal are
High refractive index material Nb ₂ O ₅ Thickness 120nm
Low refractive index material SiO ₂ thickness 120 nm
Cycle in x direction 500nm
Slow axis refractive index 1.886
Fast axis refractive index 1.837
In the case of a half-wave plate, the thickness of the entire stack is 15.8 μm
Is.

FIG. 4C is an example of a design for high definition, and the materials are common.
The dimensions of the photonic crystal are
High refractive index material Nb ₂ O ₅ Thickness 60nm
Low refractive index material SiO ₂ thickness 60nm
250 nm period in x direction
Which is half that of FIG. 2(b),
Slow axis refractive index 1.878
Fast axis refractive index 1.841
The thickness of the entire stack is 20.9 μm for a half-wave plate
Is.

The characteristics of FIG. 4B and FIG. 4C are commonly shown by the dispersion relation (or band diagram) of FIG. 4A. I will explain how to use it. In FIG. 4A, #1 indicates the TE first band (black circle) and the TM first band (white circle). These bands are almost linear when the normalized frequency L/λ is 0 or more and 0.24 or less. The meaning is, for example, the effective refractive index of TE wave and TM wave at normalized frequency = 0.1548 (upper dotted line) is the effective refractive index of TE wave and TM wave at half normalized frequency 0.0774 (lower dotted line). That is, it is almost the same as the refractive index. Therefore, even if the structure is arbitrarily reduced while maintaining the similarity ratio (2:1 in this example), almost the same properties as the medium (for example, birefringence) can be obtained. High resolution can be realized arbitrarily by keeping the similar shape and reducing the unit period of the solid. It has been found that the self-cloning method can produce a photonic crystal up to an extremely small value of p. Currently, it has a track record of commercialization up to p=80 nm, and even below that, p=40 nm is possible.

FIG. 5 is a surface view of a hierarchical distribution phase plate prism composed of a high-definition self-cloning photonic crystal as shown in FIG. 4(c), and the known metasurface phase plate shown in FIG. Is different. The surface shape of the photonic crystal type phase plate prism of FIG. 5 is qualitatively similar to that of FIG. 1, but the substance is a volume-type high-definition photonic crystal and is novel, so caution is required. The difference is that the phase plane is converted in FIG. 1 as light passes through the surface, and the phase plane is changed while waves propagate in the volume in FIG.

The photonic crystal type phase plate prism shown in FIG. 5 is different from the prior art example shown in FIG.
The number of sub-regions within one cycle in the x direction (the number of gradations in the principal axis direction) is larger, and the target phase distribution can be reproduced more properly.
The ratio between the width of the sub region and the distance between the grooves of the photonic crystal (the distance between the thick black lines and the distance between the thick white lines in the figure) is large, so that the anisotropy is more accurately realized.
High definition is achieved in the above sense.

In FIG. 5, in the approximate sense, when θ increases or decreases linearly with x, the element has a prism effect. Therefore, if the precision of the wavefront is increased by increasing the definition, the accuracy of the amplitude distribution and the phase distribution of the transmitted light realized in the prism is significantly improved.

In this structure, when the wavelength is λ and the width of the region is D, one circularly polarized light is refracted at a refraction angle λ/D radian, and the other circularly polarized light is refracted at a refraction angle λ/D radian in the opposite direction. Is 2λ/D radian. When D is selected to be about 10 wavelengths, the separation angle is about 12 degrees, which is about twice as large as the separation angle obtained with natural crystalline rutile, which is useful for downsizing optical equipment.

FIG. 5 shows the structure of a self-cloning high-definition photonic crystal. In the photonic crystal, in the three-dimensional space x, y, z, two types of transparent media having different refractive indexes alternate in the z direction on a substrate 303 having periodic pattern irregularities on the xy plane. Are stacked on. The two types of transparent media have an uneven structure corresponding to the unevenness of the substrate 303.

Multiple types of transparent bodies that form self-cloning photonic crystals are amorphous silicon, niobium pentoxide, tantalum pentoxide, titanium oxide, hafnium oxide, silicon dioxide, aluminum oxide, and fluorides such as magnesium fluoride. Is preferred. Two or more of these having different refractive indexes can be selected and used for the photonic crystal. For example, a combination of amorphous silicon and silicon dioxide, niobium oxide and silicon dioxide, tantalum pentoxide and silicon dioxide is preferable, but other combinations are also possible. Specifically, the self-cloning photonic crystal has a structure in which a high refractive index material and a low refractive index material are alternately stacked in the z direction. The high refractive index material is preferably tantalum pentoxide, niobium pentoxide, amorphous silicon, titanium oxide, hafnium oxide, or a combination of two or more of these materials. The low refractive index material is preferably silicon dioxide, aluminum oxide, a fluoride containing magnesium fluoride, or a combination of two or more of these materials.

As shown in FIG. 5, in the xy plane, a plurality of regions D are periodically and repeatedly formed in at least the x-axis direction. It is preferable that the plurality of regions D have the same length in the x-axis direction. Each area D is further divided into a plurality of sub areas in the x direction. In the example shown in FIG. 5, each region D is divided into 11 sub-regions, but may be 11 or more regions, and is preferably an odd number such as 13, 15, 17, 19 or the like. .. It is preferable that the sub regions included in each region D have substantially the same width in the x direction. The “substantially equal width” means that an error of ±2% is allowed based on the width of the sub region located in the center in the x direction.

In addition, a plurality of grooves are periodically formed in each sub region. The widths of the grooves are substantially equal. In addition, the groove is formed from end to end in the x direction in each sub region. In the region D, in the sub region located at the center in the x direction, grooves extending parallel to the x axis direction are repeatedly formed in the y direction periodically. On the other hand, in the region D, in the sub regions located at the left and right ends in the x direction, grooves extending parallel to the y direction are formed. Therefore, the angle θ formed by the grooves formed in the left and right sub-regions is 90 degrees with respect to the groove formed in the central sub-region. In such a sub region, the length of the groove is the largest and matches the effective dimension in the y direction of the entire device.

Further, between the central sub-region and the left and right sub-regions, four sub-regions are located on each of the left and right sides. A plurality of grooves are also formed periodically and repeatedly in the y direction in each sub region located between them. Further, the angles of the grooves formed in a certain sub-region are all the same. However, the angle θ of the groove of each sub-region located between them is set so as to gradually approach 90 degrees from the central sub-region toward the sub-regions at the left and right ends. For example, as described above, four sub-regions are provided between the central sub-region and the left and right end sub-regions, respectively. If the angle of the groove is 90 degrees, the inclination angle θ becomes steep by 22.5 degrees in order from the area close to the central sub area. In this way, each region D is divided into a plurality of sub-regions having the same width in the x-direction, grooves having equal angles are periodically formed in each sub-region, and the sub-regions located in the center in the x-direction are left and right. The angle of the groove monotonically increases toward the sub regions located at both ends.

Under such a premise, in each sub-region, the inter-groove unit period p (see FIG. 1) is 1/4 or less of the wavelength of incident light (for example, selected from 400 nm to 1800 nm). Becomes The lower limit of the unit period p between grooves is 40 nm. Further, in the thickness direction (z direction), the unit period of the two types of transparent media having different refractive indexes is also ¼ or less of the wavelength of light. The lower limit of the unit period in the thickness direction is 40 nm. Then, in the entire plurality of regions D, the in-plane minimum value d of the groove length (see FIG. 5) becomes 1 time or more the inter-groove unit period p described above. The upper limit of the in-plane minimum value d of the groove length is considered to be 50 times the inter-groove unit period p described above. Here, as shown in FIG. 5, since the plurality of sub-regions formed in a certain region D have the same width in the x direction, the in-plane minimum value d of the groove length in the region D is basically The length of the groove formed in the sub region located at the center of the region D. Note that the length of the groove tends to be longer as the groove is located on both left and right sides in the x direction.

As a result, the high-definition photonic crystal shown in FIG. 5 has a larger number of sub-regions within one period in the x direction (the number of gradations in the main axis direction) than the example shown in FIG. The phase distribution of can be reproduced more accurately. Further, the ratio of the width of the sub region and the distance between the grooves of the photonic crystal (the distance between the thick black lines and the distance between the thick white lines in the figure) is large, so that the anisotropy is more accurately realized. In this sense, the one shown in FIG. 5 has high definition.

FIG. 6 is a simulation result showing the relationship between the intensity of transmitted light (refracted light and straight light) and the number of sub-regions with respect to incident light when a circularly polarized Gaussian beam is vertically incident on a polarization separation element with D=5 μm. At this time, the beam has a wavelength of 1.55 μm and a diameter of 5 μm. From this graph, it can be seen that the photonic crystal type of the present invention has better characteristics than the known meta surface type. FIG. 6A shows the intensity of refracted light, which is preferably high. Therefore, it can be seen that the light intensity is improved by 0.2 to 0.3 dB even with the same number of sub regions. FIG. 6B shows the intensity of light that travels straight without being refracted, and is preferably small. Similarly, it can be seen that the number of sub-regions is improved by 3 to 7 dB. Further, since the photonic crystal type can be made finer, the number of sub regions can be made larger than that of the meta surface type while maintaining the same width D. The advantage over the meta-surface type is that the number of sub-regions can be increased to improve the accuracy of the amplitude distribution and phase distribution of the transmitted light, and gradually progresses inside the photonic crystal having a thickness of about 10 wavelengths. This is because the phase change suppresses the phase discontinuity that occurs between the sub regions.

[Curved type]
In Example 1, the superiority of the photonic crystal type polarization separation element divided into the sub regions was shown. However, there is an essential problem that the phase error generated by dividing into sub regions cannot be avoided. Further, when the number of sub-regions is increased, sufficient anisotropy cannot be obtained in the sub-regions, and there is a problem that retardance becomes small. Therefore, if the period is reduced to obtain a large polarization separation angle, the number of sub-regions must be reduced, which results in a large quantization error and deterioration of the polarization separation characteristic. In this embodiment, a method for solving this problem will be described.

The ideal angle distribution of the optical axis changes from 0 degree to 180 degrees within one cycle, and the amount of change is proportional to x. This ideal angular distribution is such that when x is between −D/2 and D/2, the pattern (convex or concave) of the photonic crystal is expressed as a curve (D/π)×log(cos(πx/D )) can be achieved. Since the tangent of this curve is the angle of the optical axis, this gives an ideal optical axis distribution. Hereinafter, the polarization separation element having such an axial orientation will be referred to as a curved type.

FIG. 2 shows a form of this embodiment. By making the pattern (convex portion or concave portion) of the photonic crystal into a curved shape, the pattern becomes sparse in the central portion within one cycle, and becomes closer to the edge and becomes denser and the pattern breaks down. Therefore, the pitch between patterns in the central portion is taken as a reference, and this is designated as p ₀ . Two patterns are merged at a position where p ₀ is below a certain threshold pitch. The pitch immediately after the merging is 2p ₀ , but since the pitch becomes denser toward the end, the merging is performed again when the length becomes equal to or less than the threshold length. By repeating the above operation, it is possible to realize an ideal optical axis distribution while changing the pitch within a certain range. When the threshold pitch is 0.5p ₀ , the pitch change range is between 0.5p _{0 and} 2.0p ₀ . That is, it is geometrically arranged so that the ratio of the maximum value and the minimum value of one of the intervals between the adjacent convex portions and concave portions is within 4 times, and the other branches and merges. In the example shown in FIG. 2, the white portion is the concave portion and the black portion is the convex portion.

As shown in FIG. 3, in the self-cloning photonic crystal, the change in effective refractive index is small with respect to the change in pitch. That is, since the amount of change in retardance is insensitive to the change in pitch, it is superior to the meta surface type. For example, if the reference pitch p ₀ preferable for the wavelength band of 1.55 μm is 600 nm, the minimum pitch threshold is 0.65 p ₀ , and the maximum pitch threshold is 1.3 p _{0, the} pitch changes between 400 nm and 800 nm, and the retardance changes. The amount is within ±10%, which is extremely small.

FIG. 7 shows a surface SEM image of the quartz substrate before film formation and the self-cloning photonic crystal formed thereon. It is designed for the wavelength band of 1.55 μm, the reference pitch is 300 nm, and the photonic crystal materials are Nb ₂ O ₅ and SiO ₂ . The threshold pitch for merging the patterns is 0.5p ₀ . From this SEM image, it is understood that by forming a curved pattern on the quartz substrate, a photonic crystal is formed along the pattern.

FIG. 8 shows the measurement result of the optical axis direction of the photonic crystal type polarization separation element. The wavelength used for the measurement was 520 nm, and the design of the photonic crystal was changed according to the measured wavelength. The reference pitch is 300 nm, the photonic crystal material is Nb ₂ O ₅ and SiO ₂ , and the film thickness is 40 nm. The layers are stacked for 15 periods, and the total thickness of the photonic crystal portion is 1.2 μm. The threshold pitch for merging the patterns is 0.5p ₀ . The polarization separation element was prepared with a period of 6, 8 and 10 μm. For comparison, a pattern divided into sub-regions is also formed on the same substrate, and the films are collectively formed. From this measurement result, it is understood that the curved element can realize the ideal axial orientation distribution indicated by the dotted line. On the other hand, in the split type, it can be seen that the axis direction changes stepwise due to the quantization error.

FIG. 9 shows measured values of split type and curved type zero-order light (unnecessary light generated by a phase error). From this, it is understood that the curved type can reduce unnecessary light. This shows that the curved type is feasible and has higher performance than the split type.

[Three-layer structure]
The prisms of the first and second embodiments operate on predetermined circularly polarized light, and the output is also circularly polarized light. However, linearly polarized light has a high utility value in an ordinary optical system. (For example, in optical communication, the laser light source, the PLC waveguide, the LN modulator, and the silicon photonics element are all in the eigenstate and in the stable operation state of linearly polarized light.) Linearly polarized light and circularly polarized light are ¼ wavelength plates. Mutual conversion is possible by passing light. Therefore, if the three-part configuration of the 1/4 wavelength plate... Prism of FIG. 7... 1/4 wavelength plate is used, a prism of linearly polarized light input and linearly polarized light output is obtained, and the utility value is increased.

In the self-cloning photonic crystal technique, a uniform quarter-wave plate is formed on a flat substrate, and a photonic crystal half-wave having the curved principal axis azimuth distribution shown in Example 2 is formed thereon. It is feasible to form a plate, and further to form a uniform quarter-wave plate on it (surface flattening is required on the way, but this can also be done by sputtering in the same apparatus).

In detail,
(1-1) Forming a uniform groove array on the substrate by nanoimprinting method (1-2) Forming a uniform quarter wave plate on it by self-cloning method (1-3) Sputtering method, etc. Then, the surface thereof is flattened (2-1), and a desired pattern is formed thereon by a nanoimprint method or the like (2-2), and the self-cloning method has the curved principal axis azimuth distribution shown in Example 2. A half-wave plate is formed (2-3) The surface is flattened by a sputtering method or the like (3-1), and a uniform groove array is formed by a nanoimprint method or the like (3-2). The order of forming a uniform quarter-wave plate by the cloning method is followed.

Hereinafter, the polarization separation element having the three-layer structure will be described with reference to FIG. A first quarter-wave plate 1003 made of a uniform self-cloning photonic crystal is formed on a substrate 1004. A half-wave plate 1002 made of a self-cloning photonic crystal having the curved principal axis orientation distribution shown in the second embodiment is formed thereon. The half-wave plate 1002 can be replaced with the self-cloning photonic crystal having the split principal axis orientation distribution shown in the first embodiment. A second quarter wave plate 1001 made of a uniform self-cloning photonic crystal is formed thereon. Like the linearly polarized light 1005, the linearly polarized light 1005 that is incident at an azimuth of 45 degrees with respect to the principal axis of the first quarter-wave plate 1003 is converted into counterclockwise circularly polarized light 1006. The left-handed circularly polarized light 1006 is converted into the right-handed circularly polarized light 1007 by the ½ wavelength plate 1002 whose principal axis directions are distributed in the plane, and the equiphase surface is inclined. As a result, the traveling direction of the right-handed circularly polarized light 1007 is tilted, enters the uniform second quarter-wave plate 1001, and exits as linearly polarized light 1008. At this time, by changing the azimuths of the principal axes of the first quarter-wave plate 1003 and the second quarter-wave plate 1001 that are uniform on the incident side and the outgoing side by 90°, the incident linearly polarized light 1005 and the outgoing light are emitted. The directions of the linearly polarized light 1008 can be made the same.

Next, FIG. 11 shows a case where the incident linearly polarized light 1101 is different from the incident linearly polarized light 1005 in FIG. The linearly polarized light 1101 enters a uniform first quarter-wave plate, is converted into right-handed circularly polarized light 1102, and is converted into left-handed circularly polarized light 1103 by a half-wave plate whose principal axis direction is distributed in the plane. In contrast to the case of FIG. 10, the equiphase surface is inclined. As a result, the traveling direction is inclined and enters the uniform second quarter-wave plate, and exits as linearly polarized light 1104.

Next, FIG. 12 shows a case where the incident linearly polarized light 1201 is incident in the same azimuth as the principal axis of the uniform first quarter-wave plate. The linearly polarized light 1201 enters the uniform first quarter-wave plate, and becomes linearly polarized light 1202 and exits. The linearly polarized light 1202 can be regarded as a combination of right-handed circularly polarized light and left-handed circularly polarized light having the same amplitude, and can be decomposed. The decomposed right-handed circularly polarized light and left-handed circularly polarized light have their traveling directions tilted as shown in FIGS. It is split into two linearly polarized lights with different 90° azimuths. At this time, the light intensities of the linearly polarized lights 1203 and 1204 are the same.

Next, FIG. 13 shows a case where a linearly polarized wave 1301 in an arbitrary direction is incident. Linearly polarized light 1301 in an arbitrary direction enters a uniform first quarter-wave plate and is converted into elliptically polarized light 1302. The elliptically polarized light 1302 can be regarded as a combination of right-handed circularly polarized light and left-handed circularly polarized light having different amplitudes, and can be decomposed. The decomposed right-handed circularly polarized light and left-handed circularly polarized light have their traveling directions inclined by the half-wave plate whose principal axis directions are distributed in the plane as shown in FIGS. It is split into two linearly polarized lights with different 90° azimuths. At this time, the balance of the light intensity of the linearly polarized light 1303, 1304 changes depending on the direction of the linearly polarized light 1301. (+45° component, -45° component)

In this way, the incident linearly polarized light is polarized and separated by the curved polarization separating element (polarization grating) using the self-cloning photonic crystal.

Even if the incident polarized light is elliptically polarized light, it is similarly polarized and separated into linearly polarized light of each component.

Here, the curved type is used for the polarized light separating portion, but it is possible to operate even if the divided type is used. In that case, the generated phase error is larger than that of the curved type.

[Lensed prism]
By combining the curved type polarization separation element of Example 2 with the quarter wavelength plate and the lens, it is possible to collect a desired polarization component while removing unnecessary polarization components. The polarization separation element may be replaced with a split type polarization separation element.

The incident light as shown in FIG. 14 is split into right-handed circularly polarized light and left-handed circularly polarized light by the polarization splitting element of Example 2, and propagates in an oblique direction. The quartz layer is transmitted and converted by the second polarization separation element arranged so that the circularly polarized light separated from the first polarization separation element is reversed, so as to propagate in parallel. After that, it is converted into linearly polarized light (vertical polarized light or horizontal polarized light) having a polarization major axis in the same direction by a region-division type quarter-wave plate whose axis orientation is changed by 90 degrees. After that, the linearly polarized light is collected by the photonic crystal lens. At this time, if unnecessary polarized light whose 90°-polarization major axis direction is different from that of the condensed light exists as crosstalk, this polarized light is not condensed, so that the polarization crosstalk and the polarization purity of the output light should be improved. You can

The prism function portion will be described below using specific parameters.
The calculation conditions are as follows.
・Wavelength 1.55 μm
・Gaussian beam with incident light radius of 2.5 μm (beam radius of commercially available high refractive index difference single mode optical fiber)
・Right-handed circularly polarized light ・Material (PBS and QWP) a-Si/SiO2
・PBS cycle 3μm
・PBS thickness 3.4 μm
・QWP thickness 1.7 μm
・Quartz thickness 50 μm
As a result of BPM analysis,
・Output light radius 3.68 μm
・Wave surface radius of curvature 25.4 μm
・Beam separation width 8.2 μm
was gotten.

Next, the lens function part will be described. The beam parameters obtained by the above analysis were analyzed under the incident conditions. The left side of FIG. 15 shows an outline of the lens.
・Lens thickness 4.8μm
Radius at which the pattern switches
2.6 μm, 3.61 μm
Effective refractive index from the center,
2.713, 2.600, 2.486
And The light is condensed in the quartz propagation layer, and the loss is estimated on the right side of FIG. 15 assuming a device such as a silicon photonics device. As an example, when the connected device has a mode field with a radius of 1.5 μm, the connection loss in the lensed prism is estimated to be about 0.95 dB.

As described above, polarization separation and light collection can be realized with a very thin composite optical element, which is 74 μm in this example. This is an order of magnitude smaller than the one using a silica-based planar optical waveguide (PLC, propagation path length of several 10 mm) for polarization separation between an optical fiber and silicon photonics.

Here, the curved type is used for the polarized light separating portion, but the operation is possible even if the sub-region type is used. In that case, the generated phase error is larger than that of the curved type.

In addition, it is preferable that the polarization separation element, the quarter-wave plate, and the lens that form the lensed prism (composite optical element) are all formed of photonic crystals. The polarization splitting element of the photonic crystal type may be either the split type of the first embodiment or the curved type of the second embodiment described above. A photonic crystal type quarter-wave plate is known. As the lens, an ordinary lens can be used, but by using a photonic crystal type lens, the thickness in the light propagation direction can be reduced.

As shown in FIG. 15, in the three-dimensional space x, y, and z, the photonic crystal type lens has the x-axis and the y-axis as the main axes of birefringence, and overlaps with a columnar central portion extending in the z-direction. And at least one peripheral portion surrounding the portion. The central part and the peripheral part are self-cloning photonic crystals. The central part has a higher effective refractive index than the peripheral part. The boundary between the central part and the peripheral part is circular or rectangular. The photonic crystal type lens guides light having an electric field in the x or y direction in the z direction and converts the spot size of the guided propagating light. Further, the peripheral portion may include a first peripheral portion surrounding the central portion and a second peripheral portion surrounding the first peripheral portion and having a lower effective refractive index than the first peripheral portion. In this way, by stacking an aggregate of three-dimensional photonic crystal minute wave plates, it functions as a vertical optical waveguide, and an optical element that collects, diverges and refracts linearly polarized light is provided. As a result, the light condensing property of the thin flat plate photonic crystal lens can be further strengthened, and further miniaturization and high light condensing function can be provided.

In the photonic crystal type lens, it is preferable that the radius of the central portion or half the length in the longitudinal direction of the xy plane of the central portion is within 10 times the wavelength of the propagating light. Further, it is preferable that the stepped refractive index distribution which approximates the quadratic parabolic n = q-p in a large range of light intensity in the xy plane (x 2 + ^{y 2).}

Lenses and prisms form the basis of all optical technologies and have utility value everywhere. For example, according to optical communication,
The lens is used to guide the light from the laser light source to the fiber or the planar optical circuit (PLC), or to couple the light in the planar optical circuit (PLC) to the modulator/switch.
The prism divides into two kinds of linearly polarized light (for example, whether the electric field is parallel or perpendicular to the reference plane) propagating through the fiber or the optical circuit, and vice versa. It is an important optical element used in a part where it is bundled (such as an optical fiber).
Further, when a circuit having polarization dependence is used, the polarization is divided into two components, and the respective lights are input to the same two circuits by aligning them with a desired light beam diameter and a predetermined polarization direction. Used for optical coupling module.
Since the propagation direction of light is reversible, the polarization splitting prism of FIG. 13 can be used in the opposite direction to be used as a polarization combining prism. In the lensed prism of FIG. 14, predetermined linearly polarized light can be made incident from the two ports at the right end, and an output combined at the left end can be obtained.
Therefore, the first, second, third, and fourth embodiments are, in that sense, a polarization separating element and a polarization combining element. Both forms can be used industrially.

Claims (7)

In the three-dimensional space x, y, z, a half-wave plate of a photonic crystal formed in the xy plane and stacked in the z-axis direction is provided,
has a single or repeated region in the x-axis direction, or
The region is divided into a plurality of strip-shaped sub-regions in the x-axis direction,
The groove direction of the photonic crystal is
Within the region, the angle with respect to the y-axis direction changes stepwise in the range of 0° to 180°, and
Within the sub-region, the angle with respect to the y-axis direction is uniform,
Light incident in the z-axis direction
right-handed circularly polarized light in the direction from the z-axis toward the x-axis at an angle,
Left-handed circularly polarized light in the direction toward the x-axis at the same angle from the z-axis,
An optical element that separates, converts, and emits light. In the three-dimensional space x, y, z, a half-wave plate of a photonic crystal formed in the xy plane and stacked in the z-axis direction is provided,
has a single or repeated region in the x-axis direction, or
The groove direction of the photonic crystal is
Is a curve, and
The angle with respect to the y-axis direction changes continuously in the range of 0° to 180°,
Light incident in the z-axis direction
right-handed circularly polarized light in the direction from the z-axis toward the x-axis at an angle,
Left-handed circularly polarized light in the direction toward the x-axis at the same angle from the z-axis,
An optical element that separates, converts, and emits light. The optical element according to claim 2, wherein
An optical element geometrically arranged so that the ratio between the maximum value and the minimum value in the area of one of the intervals between adjacent convex portions and concave portions is within 4 times, and the other branches and merges. The optical element according to claim 3, wherein
When the width of the region is D,
The optical element in which the curve is represented by y=(D/π)log(cos(πx/D))+constant. The optical element according to claim 1 or 2, wherein
The unit period between grooves of the photonic crystal is 40 nm or more and 1/4 or less of the wavelength of incident light,
An optical element whose period in the thickness direction is ¼ or less of the wavelength of incident light. The optical element according to claim 1 or 2, wherein
Laying or arranging a quarter wave plate made of a photonic crystal on one side or both sides,
An optical element that splits light incident from the z-axis direction of the optical element into two linearly polarized light beams that are orthogonal to each other. It has two optical elements according to claim 1 or claim 2,
The first optical element and the second optical element are arranged at intervals of a propagation length,
A quarter-wave plate provided after the second optical element,
A pair of lenses provided on the latter stage of the quarter-wave plate for condensing linearly polarized light and diverging the linearly polarized light orthogonal to the linearly polarized light;
Composite optical element that separates the light incident from the first optical element side into two linearly polarized light components and collects them